Disease is a dispel type that can be inflicted by some bosses and mobs. They exist both as debuffs and, in rare cases, as buffs. Disease debuffs tend to inflict various negative effects to attributes as well as damage over time, and tend to be long-lasting effects (normally between 5 and 30 minutes). Mobs that cause disease are usually only those with some indicating word in their name, like "diseased", "rabid", or "tainted". Many undead creatures cause disease.
List of diseases applied by players
Currently, death knights, druids, and priests are the only classes which can apply diseases.
Death knights notably also had an ability called that spreads any diseases they inflicted on their target to surrounding targets. There was also , a Guardian Elixir which gave the imbiber a chance to inflict a disease on the target, this has been changed to add a curse instead.
List of disease-removing abilities